The present invention relates to a high carbon steel wire for wire drawing excellent in wire-drawability and in fatigue resistance after wire drawing, which steel wire is used for, for example, bridge cables, various wires for aircraft, long rubber belts, cords for steel tires, etc., after wire drawing.
High carbon steel wire for wire drawing is generally required to withstand high-speed drawing and to have excellent fatigue resistance after wire drawing. Hard oxide type non-metallic inclusions are one:of the factors adversely affecting these properties.
Among oxide type inclusions, inclusions of a single composition such as Al2O3, SiO2, CaO, TiO2 and MgO, etc. are generally hard and inviscid. It is widely known, therefore, that it is necessary to enhance the cleanliness of molten steel and soften oxide type inclusions to manufacture a high carbon steel wire having excellent wire-drawability.
As methods to enhance the cleanliness of steel and soften inviscid inclusions, Japanese Examined Patent Publication No. S57-22969 discloses a method to manufacture a high carbon steel excellent in wire-drawability, and Japanese Unexamined Patent Publication No. S55-24961 discloses a method to manufacture an ultrafine wire. The basic concept of these technologies, however, is limited to the composition control of oxide type non-metallic inclusions in the ternary compound system of Al2O3-SiO2-MnO.
Meanwhile, Japanese Unexamined Patent Publication No. S50-71507 proposes to improve wire-drawability of a product by having the composition of non-metallic inclusions fall within the range of spessartite in the ternary compound system phase diagram of Al2O3, SiO2 and MnO, and Japanese Unexamined Patent Publication No. S50-81907 discloses a method to improve wire-drawability by decreasing harmful inclusions through the control of the Al amount added to molten steel.
Additionally, Japanese Examined Patent Publication No. S57-35243 proposes, in relation to manufacturing steel cords for tires with an inviscid inclusion index of 20 or below, to soften inclusions by injecting alloys containing one or more of Ca, Mg and REM after preliminary deoxidation by injecting a flux containing CaO with a carrier gas (inert gas) into molten steel in a ladle with the complete control of Al.
In above proposed methods, in case of reforming ternary system non-metallic inclusions, stable composition control is not easy while, in case of controlling multi-component non-metallic inclusions, the reduction of the size and number of inclusions and the security of ductility are hard to achieve and, thus, an improvement in wire-drawability and fatigue resistance after wire drawing cannot be expected. In Japanese Examined Patent Publication No. H4-8499, a high carbon steel wire having remarkably excellent wire-drawability and fatigue resistance after wire drawing has been realized by; specifying the total oxygen content within a prescribed range and controlling the amount and composition of inviscid inclusions; obtaining favorable distribution of the amount and size of inviscid inclusions by securing the reduction of the size and number of inviscid inclusions and their ductility; and softening inclusions by reforming the composition of inclusions into oxide type inclusions of multi-component system containing SiO2 and MnO and selectively containing Al2O3, MgO, CaO and TiO2.
In the invention disclosed in Japanese Examined Patent Publication No. H4-8499, a secondary deoxidizer containing Al and two or more of Mg, Ca, Ba, Ti, V, Zr and Na is added to molten steel, for the purpose of reforming inclusions into oxide type inclusions of multi-component system containing SiO2 and MnO and selectively containing Al2O3, MgO, CaO and TiO2. However, these alloys for deoxidation are expensive and thus it is desirable, for the reduction of production cost, to reduce the use of these costly alloys.
The object of the present invention is to provide a high carbon steel wire remarkably excellent in wire-drawability and fatigue resistance after wire drawing at a low cost by reducing the use of the above-mentioned expensive alloys.
Namely, the gist of the present invention is as follows:
(1) A high carbon steel wire excellent in wire-drawability and fatigue resistance after wire drawing, characterized in that; the total oxygen content is 15 to 50 ppm; among non-metallic inclusions included therein, the number of inviscid inclusions is 1.5 pieces/mm2 or less in average under the visual field of an optical microscope; among the inviscid inclusions, the number of those having a composition falling within composition A specified below accounts for more than 20% and the total number of those having a composition falling within composition A or B specified below accounts for 80% or more; and the thickness of the inviscid inclusions having a composition falling within composition A specified below is 40 xcexcm or less;
composition A: containing over 70% of SiO2,
composition B: containing 25 to 70% of SiO2, 8 to 30% of MnO, 40% or less of MgO, 35% or less of Al2O3, 25% or less of CaO and 6% or less of TiO2, and at least 5% of one or both of Al2O3 and MgO and, additionally, at least 2% of one or both of CaO and TiO2.
(2) A high carbon steel wire excellent in wire-drawability and fatigue resistance after wire drawing according to item (1), characterized in that the inclusions having composition B specified above contain 5% or less of other oxides (one or more of the oxides of V, Ba, Zr and Na and trace amount of other oxides inevitably included, hereunder referred to as xe2x80x9cother oxidesxe2x80x9d).
(3) A high carbon steel wire excellent in wire-drawability and fatigue resistance after wire drawing according to item (1) or (2), characterized in that the number of inviscid inclusions having a composition falling within composition A is 1 piece/mm2 or less in an observation visual field.
(4) A high carbon steel wire excellent in wire-drawability and fatigue resistance after wire drawing according to any one of items (1) to (3), characterized by containing 0.4 to 1.2% of C, 0.1 to 1.5% of Si and 0.1 to 1.5% of Mn in weight.
(5) A high carbon steel wire-excellent in wire-drawability and fatigue resistance after Wire drawing according to any one of items (1) to (3),:characterized. by containing, in weight, 0.4 to 1.2% of C, 0.1 to 1.5% of Si and 0.1 to 1.5% of Mn, additionally P and S controlled not to exceed 0.02%, respectively, and one or more of 0.05 to 1.0% of Cr, 0.05 to 1.0% of Ni, 0.05 to 1.0% of Cu, 0.001 to 0.01% of B. 0.001 to 0.2% of Ti, 0.001 to 0.2% of V, 0.001 to 0.2% of Nb, 0.05 to 1.0% of Mo and 0.1 to 2% of Co.
Here, inviscid inclusions denote inclusions whose length or thickness is 5 xcexcm or more and the length (1) and the thickness (d) of each of them satisfy the formula 1/d xe2x89xa65, under optical microscope observation of a longitudinal section including the center line of a wire.
It is generally known that, when inclusions contain the oxides of simple composition or specific oxides in high quantity, they are hard and their plasticity is poor. The most significant feature of the present invention is a discovery of the fact that the inclusions having a high content of SiO2 are softer than those having high content of Al2O3 and MgO and that, even if inclusions having high content of said SiO2 account for more than 20%, the wire-drawability and fatigue resistance after wire drawing of a steel wire are not adversely affected so long as the thickness (d) of said inclusions is controlled not to exceed 40 xcexcm.
When the total oxygen content of steel is high, blowholes are generated during the solidification of molten steel causing surface defects and the amount of inviscid inclusions increases-in a steel wire having a total oxygen content exceeding 50 ppm. For this reason, the upper limit of the total oxygen content is set at 50 ppm. On the other hand, though it is easy to reduce the total oxygen content to 15 ppm or less when a strong deoxidizer such as Al and Mg is used abundantly, a total oxygen content of 15 ppm or more is required for the composition control of inviscid inclusions in a steel wire of the present invention. A yet preferable range of the total oxygen content is from 17 to 40 ppm. Further, when the total oxygen content is below 15 ppm or more than 50 ppm, the service life of the wire drawing dies deteriorates drastically and, for this reason, the range of the total oxygen content is set at 15 to 50 ppm.
The amount of inviscid inclusions among oxide type non-metallic inclusions in a steel wire affects its wire-drawability and fatigue resistance after wire drawing. From this viewpoint, for the present invention too, it is necessary to reduce the amount of inviscid inclusions to as little as possible. By controlling the number of inviscid inclusions to 1.5 pieces/mm2 or less, excellent wire-drawability and fatigue resistance after wire drawing can be obtained through combined effects with the other requirements claimed herein. When the number of inviscid inclusions exceeds 1.5 pieces/mm2, the wire disconnection rate rises markedly high and the die service life becomes shorter. It is preferable yet to control the number of the non-metallic inclusions to 1.0 piece/mm2 or less.
In conventional technologies, inviscid inclusions are softened by compounding the composition of the inclusions. In those technologies, the SiO2 content of inclusions is specified to be 70% or less because it is understood that hard SiO2 inclusions form when SiO2 concentration exceeds the percentage figure.
The present inventors have discovered, as a result of intensive studies, that, even if inviscid inclusions have a high SiO2 content, they do not cause any harm even in a succeeding wire drawing process so long as their size is small. It is true that SiO2 type inclusions are hard, but they are softer than MgO or Al2O3 type inclusions. Therefore, wire-drawability and fatigue resistance after wire drawing are sufficiently good so long as the size of the inclusions is controlled to dxe2x89xa640 xcexcm. It is more preferable to control the size of inviscid inclusions having a high SiO2 content to dxe2x89xa620 xcexcm.
In the present invention, composition B denotes the composition range of inclusions which are sufficiently soft and rendered harmless by being broken down and minutely dispersing during wire drawing, while composition A denotes the composition range of inclusions having a higher SiO2 content than the inclusions conforming to the composition B. It has been specified that the number of inviscid inclusions conforming to composition A accounts for 20% or more and the total number of those conforming to composition A or B accounts for 80% or more.
The reason why the total number of inclusions conforming to composition A or B must account for 80% or more is that inclusions having composition not falling within any of compositions A and B are, for example, MgO and Al2O3 type inclusions and they are hard, and that these hard inclusions deteriorate wire-drawability and fatigue resistance after wire drawing when their proportion exceeds 20%.
Further, the reason why the number of inclusions conforming to composition A must account for 20% or more is that inclusions conforming to composition A increase as the addition amount of ferroalloys of Ca, Al, Mg and Ti in molten steel is decreased, and that, when the addition amount of these ferroalloys is decreased to the extent of raising the proportion of inclusions conforming to composition A to 20% or more, the cost reduction effect, which is an object of the present invention, can be achieved.
In the present invention, the range of the composition B is defined as follows:
{circle around (1)} containing 25 to 70% of SiO2, 8 to 30% of MnO, 40% or less of MgO, 35% or less of Al2O3, 25% or less of CaO and 6% or less of TiO2, and at least 5% of one or both of Al2O3 and MgO, and additionally at least 2% of one or both of CaO and TiO2,
{circle around (2)} containing 5% or less of other oxides (one or more of the oxides of V, Ba, Zr and Na and trace amount of other oxides inevitably included, hereunder referred to as xe2x80x9cother oxidesxe2x80x9d).
The reason why the range of composition B is limited will be explained hereunder.
To reduce the number of inviscid inclusions and soften them, which is an object of the present invention, the combination of oxide compositions in a multi-component system as specified above is required. One combination is oxides of a quaternary or higher compound system containing SiO2 and MnO firstly and inevitably, then one or both of Al2O3 and MgO, and additionally one or both of CaO and TiO2. Another combination is oxides of a quaternary or higher compound system containing 5% or less of the other oxides in addition to above oxides. Here, the addition of 5% or less of the other oxides contribute to softening the inviscid inclusions yet further. The steel according to the present invention does not fail to be a steel wire excellent in wire-drawability and fatigue resistance after wire drawing if inviscid inclusions conforming to composition B have either of the combinations according to the present invention.
When SiO2 content is below 25%, a good combination with the other oxides as inclusions of multi-component system oxides cannot be obtained. The range of SiO2 content of over 70% coincides with the range of composition A, which has conventionally been avoided as the one where hard inclusions are formed.
Since MnO is displaced or compounded by the oxidation with Al and Mg, 30% or more of MnO is not formed. When its content is below 8%, on the other hand, the inviscid inclusions become hard. For this reason, the range of MnO is set at 8 to 30%.
With MgO content exceeding 40%, hard MgO inclusions are formed and thus its content is limited to 40% or less. A preferable range is from 5 to 25%.
With Al2O3 content exceeding 35%, well-balanced combination of multi-component system oxides is disturbed, causing the other oxide elements in inclusions to relatively lower, resulting in the formation of hard inclusions. The upper limit of Al2O3 to avoid this problem is 35%, more preferably 25%.
With regard to the combination of Al2O3 and MgO, in the manufacture of a steel wire according to the present invention, wherein SiO2 type oxides suspended in molten steel are combined with Ca, Mg and Al, etc. during a secondary deoxidation process to form compound inclusions, inviscid inclusions become soft and are rendered harmless when the total content of one or both of Al2O3 and MgO in inviscid inclusions formed in a steel wire is 5% or more. For this reason, the lower limit of one or both of Al2O3 and MgO is set at 5%.
With regard to CaO, when the CaO content is high, generally speaking, spherical inviscid inclusions are formed. However, when CaO content is 25% or less and the inclusions are of multi-component system as in the present invention, CaO also contributes to decreasing the hardness of oxide type inclusions and reducing the number of inviscid inclusions. The upper limit of CaO content is, therefore, set at 25%. A more preferable content range of CaO is from 1 to 20%.
Ti is an element generally used for the control of austenite crystal grain size. However, it is also effective for softening non-metallic inclusions of multi-component system oxides like in the present invention. It is particularly effective for softening when TiO2 content is 6% or less in the inviscid inclusions of the multi-component system. Hence, the TiO2 content range is set at 6% or less. A more preferable range is 4% or less.
With regard to the combination of Cad and TiO2, when the content of one or both of them is 2% or more, the inviscid inclusions are softened yet further.
Lastly, the content of the other oxides limited to 5% or less is described hereunder.
The composition described above is essential for obtaining the multi-component system inviscid inclusions according to the present invention. Further, V, Ba, Zr and Na, etc. are added in addition to secondary deoxidation elements. These oxides and other oxides such as oxides of Cr and K, etc. inevitably included in steel bin very small quantities are collectively called the other oxides. When the content of the other oxides is 5% or less, they contribute to softening inviscid inclusions. For this reason, the upper limit of the combined content of one or more of the other oxides is set at 5%.
Now, combinations of the oxides described above are explained hereunder.
Firstly, the reason is given as to why SiO2 and MnO are indispensable in any case.
The inviscid inclusions comprising multi-component system oxides according to the present invention can be obtained, as described in examples, by forming a deoxidation product of SiO2+MnO at primary deoxidation and, thereafter, forming a compound SiO2 deoxidation product at secondary deoxidation. Thus, as a matter of course, SiO2 and MnO, which form the basis of the deoxidation products, must not fail to exist in inviscid inclusions.
Then, the combination of Al2O3 and MgO is described hereunder.
As one of the deoxidation technologies to form non-metallic inclusions of multi-component system oxides according to the present invention, an important technology is the utilization of strong deoxidation effects of Al and Mg and the coagulation and floating effects of said inclusions in molten steel. With regard to said inclusions remaining in molten steel after refining of molten steel, there is a relationship between Al2O3 and MgO in the same refined molten steel that, in the composition range of inviscid inclusions according to the present invention, MgO content tends to be low when Al2O3 content is high and, reciprocally, Al2O3 content tends to be low when MgO content is high. For this reason, the present invention prescribes that one or both of Al2O3 and MgO must be contained.
Next, a reason is given to explain why it is. prescribed that one or both of CaO and TiO2 must be contained.
The non-metallic inclusions of multi-component system oxides like those according to the present invention show widely varied composition changes depending on deoxidation conditions. In this background, one or both of CaO and TiO2 must be present in inviscid inclusions, especially in order to reduce the number of inviscid inclusions of multi-component system inclusions and soften them.
An important point of the present invention is to control the size of inviscid inclusions conforming to composition A so that dxe2x89xa640 xcexcm holds true. This is because the inclusions conforming to composition A do not hinder the inclusion softening effect when the formula dxe2x89xa640cm is satisfied, although they are somewhat harder than the inclusions whose composition falls within composition B.
Large inclusions whose d exceeds 40 xcexcm are mainly ladle deoxidation products formed in molten steel in a ladle after deoxidation. When compound deoxidation involving Ca, Al, Mg and Ti is carried out so that most inviscid inclusions have compositions conforming to composition B as in the present invention, said deoxidation products in a ladle are softened, and most of the large inclusions having d exceeding 40 xcexcm are stretched to satisfy l/d greater than 5. In this case, since most of those SiO2-rich inclusions conforming to composition A are those formed during steel solidification, they cannot grow large and the formula dxe2x89xa640 xcexcm remains true. Thus, d of the inviscid inclusions whose composition falls within compositions A and B can be controlled not to exceed 40 xcexcm.
As explained above, it is necessary in the present invention to control the number of inviscid inclusions to 1.5 pieces/mm2 or less. By the present invention wherein a compound deoxidation is carried out so that the total number of inviscid inclusions conforming to composition A and those conforming to composition B is 80% or more, it is possible, as a consequence, to stably maintain the number of inviscid inclusions at 1.5 pieces/mm2 or less. Preferably, by controlling the number of inviscid inclusions to 1.0 piece/mm2 or less, the wire-drawability and fatigue resistance after wire drawing of a steel wire stabilize.
The present invention can secure excellent wire-drawability and fatigue resistance after wire drawing by controlling the composition, size and number of inclusions as described above. In addition, the service life of wire drawing dies can be extended by the present invention by reducing the number of inviscid inclusions conforming to composition A to 1.0 piece/mm2 or less in average, and more preferably, to 0.5 piece/mm2 or less.
As described above, the present invention achieves excellent results in the applications where wire-drawability and fatigue resistance after wire drawing as severe as conventional cases are required. Recently, however, larger diameter cords are used in some tire cord applications, wherein the required wire-drawability is a little more relaxed than before. With regard to the service life of wire drawing dies, also, improvements in lubrication and other factors have made it possible to continue drawing operations not affected by deterioration in inclusion levels in steel materials. The super-clean steel according to the present invention has an excellent effect especially in these applications.
The specification of the steel chemical composition according to the present invention is explained hereunder. Killed steels for piano wire rods and hard steel wire rods under the Japanese Industrial Standard (JIS) G3502 and G3506 are widely used as the steels for high carbon steel wires. On the basis of these JIS steel grades and in consideration of ease of manufacturing and actual applications, the present invention specifies steel chemical composition, in weight, as follows: said steel containing 0.4 to 1.2% of C, 0.1 to 1.5% of Si and 0.1 to 1.5% of Mn, and containing one or more of 0.05 to 1.0% of Cr, 0.05 to 1.0% of Ni, 0.05 to 1.0% of Cu, 0.001 to 0.01% of B, 0.001 to 0.2% of Ti, 0.001 to 0.2% of V, 0.001 to 0.2% of Nb, 0.05 to 1.0% of Mo and 0.1 to 2% of Co as required.
C is an economical and effective element to strengthen steel, and 0.4% or more of it is required to obtain the strength required for a hard steel wire. When its content exceeds 1.2%, however, the ductility of steel decreases, resulting in embrittlement and difficulty in secondary working. For this reason, the content is set at 1.2% or less.
Si and Mn, on the other hand, are necessary for deoxidation and the control of the composition of inclusions. Either of them is ineffective when added below 0.1%. Both the elements are also effective for strengthening steel, but steel becomes brittle when either of them exceeds 1.5%.
Cr has to be controlled within a range from 0.05 to 1.0% because the least necessary amount for securing its effect to refine pearlite lamella and enhance steel strength is 0.05% and thus the addition of Cr in the amount of 0.05% or more is desirable. However, when added beyond 1.0%, ductility deteriorates. For this reason, the upper limit is set at 1.0%.
Ni strengthens steel through an effect similar to that of Cr, hence the addition amount of 0.05% or more, where the effect is demonstrated, is desirable, but its content has to be 1.0% or less so as not to cause the deterioration of ductility.
Since Cu improves scale properties and corrosion fatigue properties of a wire, the addition amount of 0.05% or more, where the effect is demonstrated, is desirable, but its content has to be 1.0% or less so as not to cause deterioration of ductility.
B is an element to enhance hardenability of steel. In case of the present invention, it is possible to enhance steel strength by adding B, but its excessive addition deteriorates steel toughness through increased boron precipitation, and for this reason, its upper limit is set at 0.01%. Too small an addition amount of B does not bring about any effect, and hence its lower limit is set at 0.001%.
Ti, Nb and V enhance the strength of a steel wire through precipitation hardening. None of them is effective when added below 0.001%, but they cause precipitation embrittlement when added beyond 0.2%. For this reason their respective contents have to be 0.2% or less. Addition of these elements is also effective for fining xcex3 grains during patenting.
Mo is another element to enhance steel hardenability. In case of the present invention, it is possible to enhance steel strength by adding Mo, but its excessive addition raises steel hardness excessively resulting in poor workability and, for this reason, the range of its addition amount is specified as 0.05 to 1.0%. Co enhances steel ductility by suppressing the formation of pro-eutectoid cementite of supereutectoid steel.
In addition, in high carbon steels, it is preferable to control the content of each of P and S to 0.02% or less since either of them deteriorates not merely wire-drawability but also ductility after wire drawing.
Note that the present invention can be applied not only to a steel wire but also to any hot-rolled steel product.